Going For Broke

Of all the things engineers solve in their careers, the most challenging is material fatigue and breakage. No matter what product a customer markets or the level of sophistication and imagination echoing through their halls, the question is the same … “why did the spring break?” The true answer is not why the material failed but what caused it to fail.

I am not a metallurgist and don’t analyze fractures or discuss grain structure. My onus is to understand what the application is doing to the design. The single most important purpose of any spring is to provide a force and return to a height to produce that same force again.

While that force is being produced, the material may be taking a pounding. A spring in a dynamic application means it’s being preloaded to a height, pressed to another height, and then returning to the first height with a certain period of time. From a springmakers perspective, the question becomes what’s happening between those two points? I can calculate the corrected stresses needed to predict cycle life, but what is really going on between points A and B?

The first step is to always examine the part that left the plant. High tensile materials like Chrome Silicon alloys perform well when their surface stays pristine. The notch sensitive nature of high hardness materials serves as a dual edged sword – the high tensile is great for high life but is not forgiving if the surface integrity is breached. So look for any tool marks, nicks, dings, burrs …anything that could touch or scratch the material surface. Be certain the very process needed to form the spring is not adding surface anomalies. Also, don’t discard any finishes or coatings as a potential culprit. Plated parts could have suffered hydrogen embrittlement which is a death knoll for high hardness alloys.

The second step is to find out what’s really going on in the application. Has the part performed well for years with no failure? Have the mating parts changed in any way – different geometry, lower or higher hardness? Has the deflection increased in any way? Has the bore or guiding shaft been altered ? One of the most common practices amongst manufacturers is to take a spring from a present inventory and try it in a new application. In other words, the part was not designed for the application for life but it performs in the new application. This saves the manufacturer time and money for testing the new device. But, if the spring is providing more force or larger deflection than it was in it’s previous role, the stresses may not allow high life.

Another factor that often goes unconsidered is temperature. Both high heat and low temps can alter material properties and lower their performance capabilities. If extreme temperatures are In the scheme of things, exotic materials may be needed to be sure the stress can be absorbed.

The fact is, spring material in compression and extension applications is twisting – stressed in torsion. Any material breach caused by either manufacturing or the application can cause early failure. Where and when is the offending breach being introduced? A material analysis can tell you the particulars of the break science but does answer not what caused the failure, specifically. Until the source of the failure is discovered and improved, history will repeat itself.

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